The invention relates to using a delta-doped charge-coupled device (CCD) to determine the energy of a low-energy particle.
A charge-coupled device (CCD) includes an array of coupled electronic gates, such as metal-oxide-semiconductor field effect transistors (MOSFETs), that together convert optical or particle energy into an electronic signal. CCDs are used in a wide variety of applications, including digital imaging systems such as digital cameras.
FIG. 1A shows the general structure of a thinned CCD 10, which commonly is used for ultraviolet light detection. The CCD 10 includes a semiconductor core 12 comprising, e.g., lightly doped silicon, onto which a thin insulating layer 14, e.g., a layer of silicon oxide, is formed. An oppositely charged dopant layer 15, e.g., n-type dopant, may be implanted at the front surface 24 of the semiconductor core 12 to form a "buried channel" CCD, which is described below. A conductive gate 16 is formed on the front surface 18 of the insulating layer 14 to apply an electric potential to the device. Typically, the back surface 20 of the semiconductor substrate 12 includes a thin, insulating native oxide layer 22 that forms naturally on the semiconductor's back surface 25.
In operation, the conductive gate 16 is biased with respect to the back surface 20 of the semiconductor substrate 12 by a voltage supply V. As photons or particles strike the device 10 through its back surface, electron-hole pairs form in the substrate core 12. Depending on whether the semiconductor is p-type or n-type, the electrons or the holes migrate toward the semiconductor-oxide surface 24, where they accumulate in a "collection well" 26 (FIG. 1B) that develops in the semiconductor 12 near the semiconductor-oxide surface 24. The implanted layer 15 creates a buried channel where collected charge accumulates in the semiconductor core 12 a given distance below the insulating layer 14.
Incident energy from photons or particles is converted into charge in the semiconductor core, and the charge accumulates in the collection well during a given integration period. The amount of charge collected in the well 26 during the integration period is generally proportional to the total energy of the particles penetrating the semiconductor 12 during the integration period. The efficiency of the conversion of energy to charge depends on the energy-dependant interaction of photons or particles in the CCD structure. Therefore, different CCD structures can have markedly different efficiencies. The CCD 10 generates an output signal by serial measurement of the charge collected in each pixel during the integration period.
The thickness of the semiconductor core 12 in a typical thinned CCD 10 is 8-15 .mu.m, which allows the thinned CCD to detect some particles striking its back surface 25. For example, a typical thinned CCD can detect electrons having kinetic energies greater than about 10 keV. The sensitivity of the thinned CCD to these low-energy particles is limited, however, by a "dead layer" caused by the presence of a "potential well" 28 (FIG. 1B), which forms near the substrate's back surface 25 as a result of charge trapped in the native oxide layer 22. Particles moving with kinetic energies below a certain level do not penetrate far enough into the CCD to overcome the potential well.
Backside surface treatment technology has been used to alter the CCD structure and thus to reduce the effects of the potential well 28. These techniques include UV-induced adsorption of negative ions on the native oxide surface, deposition of a conductive layer over the oxide, and introduction of a thin p+layer by ion implantation. Backside treatment has improved the particle detection capabilities of CCDs, but the utility of CCDs as particle detectors is limited by the CCD structures. For example, using conventional detectors, detection of electrons is limited to particles with energies above 1 keV, and detection of protons is limited to particles with energies above 10 keV.